MONTPELIER — Economists for the state can forecast with surprising accuracy the tax revenues expected to flow into the treasury over the next fiscal year.

But predicting one of the largest sources of funds to Vermont’s $5.2 billion budget — federal revenues — has proven a more difficult task. And despite a recent analysis done by experts for the Legislature, pinning down the federal funds on which the state so heavily relies is still something of a guessing game.

“There are just so many unknowns at this point,” said Steve Klein, chief fiscal officer at the Legislature’s Joint Fiscal Office.

“And there are a lot of political choices that are going to get made that are totally outside our control.”

Official state revenue forecasts, the most recent of which arrived Tuesday, are historically reliable, deviating by only about 2 percent on average over actual revenues. That record of success gives lawmakers a high degree of confidence in the $1.56 billion in general fund revenues forecast to flow into Montpelier over the next 12 months.

Getting similarly reliable estimates for money flowing from the federal government is more difficult. But Klein said one thing is clear: risk for Vermont is to the downside.

In a report titled “Federal Funds — Reductions Today and Tomorrow,” Klein surveyed the federal landscape in an attempt to divine how decisions in Washington, D.C., will affect Vermont. From health care and transportation to grants that support human services, Klein said Tuesday that federal funds coming into Vermont will likely shrink in the coming years.

“And one area where that could be extremely noticeable is in transportation,” Klein said.

The reauthorization of the five-year federal transportation bill will likely overhaul a state-by-state allocation formula that has until now been favorable for the state. Vermont gets $2.85 cents from the feds for every $1 in gas tax it sends to the federal treasury, a ratio Klein said he expects will narrow in the next five-year bill.

Projected declines in federal discretionary spending, according to Klein, also imperil some of the federal dollars coming into